The present invention relates to a high-strength steel pipe having improved formability and burst resistance when the steel pipe is formed by a UOE manufacturing method.
The group of processes for producing a steel pipe by a UOE method is generally composed of the processes of: C-forming (pressing) of a steel sheet, U-forming (pressing), O-forming (pressing), seam welding and pipe expansion, as shown in FIG. 1. In the C-forming process, beveling is applied to both the edges of a steel sheet and then bending is applied to the vicinities of the edges of the steel sheet. The bending is mostly applied with press forming, but it is also possible to form bent portions in the vicinities of the edges of a steel sheet with roll forming as disclosed in Japanese Unexamined Patent Publication No. S61-279313. The steel sheet after being subjected to the C-forming is then formed into a xe2x80x9cU-shapexe2x80x9d in a U-forming process, and thereafter formed into a tubular shape in an O-forming process. After that, both the edges of the steel sheet formed into a tubular shape, whose bevel ends are in butting relation with each other, are seam-welded in a seam welding process. At this stage, a pipe closed in the circumferential direction is formed for the first time and, then, the pipe is subjected to pipe expansion using a pipe expansion device called an expander in a pipe expansion process for obtaining a better tubular shape, namely for improving the roundness of the pipe. As the methods of expanding a pipe, there are two methods; the mechanical pipe expansion method wherein a deformation is imposed forcibly from the interior of a pipe towards the exterior thereof, and the hydraulic pressure pipe expansion method wherein a hydraulic pressure is imposed in the interior of a pipe. At present, the former method is mostly employed. Note that, though there is the method of reducing the diameter of a pipe from the exterior thereof for improving the roundness of the pipe in contrast with the above pipe expansion methods, this method is differentiated from the UOE method.
In the past, in the UOE method for producing a steel pipe, many inventions have been produced for improving formability such as roundness, the capacity of existing facilities and the formability of a pipe having a heavy wall thickness by specifying the forming conditions in each of the processes of C-forming, U-forming, O-forming and pipe expansion.
For example, in the forming method of C-pressing, Japanese Patent Application No. H8-294724 discloses a method of reducing peaking (a positive deviation from the concentric circle at a weld) and making it possible to form a heavy thickness material or a high-strength material by prescribing a specific relation to the forming length, the yield strength of a sheet material and the thickness thereof in the C-forming process, without increasing the capacity of C-pressing and/or O-pressing.
Further, Japanese Unexamined Patent Publication Nos. H9-239447 and H10-211520 disclose that bad shapes can be improved even within the capability of existing facilities by: controlling the length of the bending region to 3.5 times or more the sheet thickness or controlling the length of the remaining straight portion to 1.5 times or less the sheet thickness when C-forming is applied; and, by so doing, restricting peaking (a protrusion at an abutting portion in this technology to 2 mm or less. Yet further, Japanese Patent No. 1135933 discloses a technology that enables the shape of a steel pipe to be improved by controlling the ratio between the radius of curvature at C-pressing (the radius of curvature before O-pressing) and the radius of curvature of the steel pipe within the range from 0.8 to 1.2 and, by so doing, reducing peaking. As technologies developed by the forming conditions in C-pressing as disclosed above, there have been proposed the technologies disclosed in Japanese Unexamined Patent Publications No. S55-14724, No. S59-199117 and No. S60-92015.
In addition, as a technology for improving the formability in O-pressing, there is the technology of reducing peaking by forming a heteromorphic portion at the center of a die caliber in the longitudinal direction as disclosed in Japanese Patent No. 1258977. Moreover, there are other technologies of improving O-pressing as proposed in Japanese Unexamined Patent Publications No. H9-94611 and No. S53-112260.
Further, as a technology of correcting roundness and bending by devising a pipe expansion process, there is the technology of applying pressing several times while the relative positions of a caliber and a material to be formed are changed as disclosed in Japanese Unexamined Patent Publication No. H03-94936. As other technologies, there are the technologies of improving roundness in relation to pipe expansion as proposed in Japanese Unexamined Patent Publication Nos. S57-94434 and S61-147930.
In recent years, the importance of a line pipe has been increasing still more as a means of long distance transportation of crude oil and natural gas. In particular, in order (1) to improve a transportation efficiency by applying a higher pressure and (2) to improve a field construction efficiency by reducing the outer diameter and weight of a line pipe, the needs for a high-strength line pipe exceeding X100 (760 N/mm2 or more in tensile strength) have been getting stronger. To cope with these needs, in recent years, a technology of applying TMCP even to the production of a steel sheet exceeding 760 N/mm2 in tensile strength, which has been difficult so far (refer to Japanese Unexamined Patent Publication No. H8-199292) has been developed.
In the meantime, as the strengthening of a line pipe advances, it has been clarified that the softening of a heat affected zone (a HAZ), which has scarcely been considered until now as a problem, when a medium- or low-strength material of about 700 N/mm2 in tensile strength has been welded with submerged arc welding or the like, advances and the critical plastic strain, at which ductile cracking starts to occur during the forming of a sheet material, lowers when a high-strength material exceeding 850 N/mm2 in tensile strength is used. Therefore, when a line pipe exceeding 850 N/mm2 in tensile strength is formed, problems such as cracking and rupture at a weld in a pipe expansion process after seam welding and brittle rupture at a seam weld when an obtained steel pipe product is subjected to an internal pressure load occur. These problems did not appear when a conventional medium- or low-strength steel pipe was produced.
The object of the present invention is, in view of problems in the existing technologies as stated above, to provide a method of producing a high-strength steel pipe so excellent in formability as not to incur cracking and rupture at a weld in a pipe expansion process when such a high-strength steel pipe, for line pipe use, exceeding 850 N/mm2 in tensile strength is produced and so excellent in burst resistance as not to incur brittle rupture at a seam weld even when the steel pipe product is subjected to an internal pressure load during its service.
The present invention has been accomplished for solving the above-mentioned problems and the gist of the present invention is as follows:
(1) A high-strength steel pipe excellent in formability, characterized in that, when a high-strength steel pipe exceeding 850 N/mm2 in tensile strength is produced by a UOE method, the ratio (R/r) of the average radius of curvature in the range of 120 mm in the circumferential direction including the weld of the steel pipe before pipe expansion in a pipe expansion process (R) to the radius of the steel pipe after pipe expansion (r) is 0.65 to 2.0.
(2) A high-strength steel pipe excellent in formability and burst resistance, characterized in that, when a high-strength steel pipe exceeding 850 N/mm2 in tensile strength is produced by a UOE method, the ratio (R/r) of the average radius of curvature in the range of 120 mm in the circumferential direction including the weld of the steel pipe before pipe expansion in a pipe expansion process (R) to the radius of the steel pipe after pipe expansion (r) is 0.90 to 2.0.
(3) A high-strength steel pipe excellent in formability, characterized in that, when a high-strength steel pipe exceeding 850 N/mm2 in tensile strength is produced by a UOE method, the absolute value of the strain in the circumferential direction at a point 4 mm distant from each of the toes of the weld during pipe expansion is 4% or less.
(4) A high-strength steel pipe excellent in burst resistance, characterized in that, when a high-strength steel pipe exceeding 850 N/mm2 in tensile strength is produced by a UOE method, the absolute value of the strain in the circumferential direction at a point 4 mm distant from each of the toes of the weld during pipe expansion is 2.5% or less.
(5) A high-strength steel pipe excellent in burst resistance, characterized in that, when a high-strength steel pipe exceeding 850 N/mm2 in tensile strength is produced by a UOE method, the peaking amount before pipe expansion satisfies the expression (1) and at least the height of the shrinkage allowance of the weld metal at the inner surface is 2.0 mm or less,
xe2x88x921.5 mmxe2x89xa6peaking amount (mm)xe2x89xa616/pipe wall thickness (mm)xe2x80x83xe2x80x83(1).
(6) A high-strength steel pipe excellent in burst resistance according to the item (5), characterized in that the change in the peaking amount from before pipe expansion to after pipe expansion satisfies the expression (2),
xe2x88x921.5 mmxe2x89xa6change in peaking amount (mm)xe2x89xa61.0 mmxe2x80x83xe2x80x83(2).
(7) A high-strength steel pipe excellent in burst resistance, characterized in that, when a high-strength steel pipe 900 N/mm2 or more in tensile strength is produced by a UOE method, the Vickers hardness of the base metal of the steel pipe Hv, the minimum Vickers hardness at the HAZ Hz, the pipe wall thickness t, and the amount of peaking deviated from the perfect circle in the range of 120 mm in the circumferential direction including the weld of the steel pipe before pipe expansion in a pipe expansion process xcex4 satisfy the expression (3),
(1+0.005t|xcex4|) Hz less than 0.03584Hv2xe2x88x9225.34Hv+4712xe2x80x83xe2x80x83(3).
(8) A high-strength steel pipe excellent in burst resistance according to the item (7), characterized in that the peaking amount xcex4 satisfies the expression (4),
xe2x80x83|xcex4| less than 40/txe2x80x83xe2x80x83(4).